This invention relates to the manufacture of anhydrous alkali dithionites by reacting an alkaline formate, an alkali metal agent, and sulfur dioxide in an alcohol/water solvent. It particularly relates to improving this process by removing troublesome impurities from the reaction mixture while the reaction is occurring.
In the process for the manufacture of alkali metal dithionites from an alkali metal salt of formic acid, an alkali metal hydroxide, carbonate, or bicarbonate, and sulfur dioxide, the pertinent chemistry is believed to be as shown in the following equations, using sodium formate and sodium hydroxide for illustration: EQU 2NaOH+2SO.sub.2 .fwdarw.Na.sub.2 S.sub.2 O.sub.5 +H.sub.2 O, (1) EQU 2HCOONa+2SO.sub.2 +H.sub.2 O .fwdarw.2HCOOH+Na.sub.2 S.sub.2 O.sub.5, (2) EQU 2HCOOH+2Na.sub.2 S.sub.2 O.sub.5 .fwdarw.2Na.sub.2 S.sub.2 O.sub.4 +2CO.sub.2 +2H.sub.2 O. (3)
According to these equations, and assuming that an excess of sodium metabisulfite is present, one mol of sodium dithionite should be produced via Equation (3) for each one mol of formic acid generated as shown in Equation (2).
In practice it is found that substantially less than one mol of sodium dithionite is produced per mol of formic acid. Empirically it has been established that approximately 0.8 mol of sodium dithionite is produced per mol of formic acid.
One reason for this yield deficiency is that formic acid in the alcohol reaction medium used for the process undergoes a certain degree of esterification: EQU HCOOH+CH.sub.3 OH.revreaction.HCOOCH.sub.3 +H.sub.2 O. (4)
The alcohol used for the reaction medium is recovered for re-use via distillation. The methyl formate in the alcohol should be similarly recovered. If the methyl formate recovery were 100% efficient, no yield loss would be occasioned by the chemistry of Reaction (4). In practice, however, it has been found that methyl formate losses do occur, owing principally to its very high volatility. Some loss occurs directly from the reactor, as the methyl formate is carried out by the effluent carbon dioxide. Other losses occur in the act of filtering, washing, and blowing the product filter cake. Still more loss is occasioned by the distillation process. The severity of these various losses will be determined by the perfection of the design and operation of the equipment used to carry out each of these functions.
A second source of yield deficiency is chemical decomposition of sodium dithionite after it is made. While several modes of sodium dithionite decomposition are possible, the predominating loss results in the formation of sodium thiosulfate and other unidentified sulfur compounds which are found in the reactor throughout the course of the reaction: EQU 2Na.sub.2 S.sub.2 O.sub.4 .fwdarw.Na.sub.2 S.sub.2 O.sub.3 +Na.sub.2 S.sub.2 O.sub.5. (5)
It has also been found that the sodium thiosulfate forming reaction is auto-catalytic with respect to sodium thiosulfate. That is, as the concentration of sodium thiosulfate increases in the reactor, its rate of formation similarly increases. This increasing rate of formation may be owing to thiosulfate itself or to the accompanying sulfur compounds. A number of factors are known to influence the rate of sodium thiosulfate formation, principally the reaction temperature, the pH, and the water-to-alcohol ratio in the reaction medium. Despite all efforts to optimize these various conditions and minimize the loss of sodium dithionite, this loss continues to be a principal cost in the manufacture of sodium dithionite.
Japanese Patent Publication No. 28,397/75 teaches a process for manufacturing anhydrous sodium dithionite in an alcohol/water solvent from sodium formate, an alkali compound, and sulfur dioxide, followed by filtering the sodium dithionite crystals from the mother liquor. The publication discloses a method for recycling the reaction filtrate with reduced distillation of the filtrate by treating the filtrate with 1-to-4-fold excess on a molar basis of ethylene oxide, propylene oxide, or a mixture thereof over the amount of sodium thiosulfate contained in the reaction filtrate and by allowing the reaction mixture to stand for several hours at room temperature. The reaction filtrate is combined with the methanol used in washing the separated crystals of sodium dithionite to form a mixture of which a part is distilled to recover the methanol and isolate the addition product, which is discarded, of sodium thiosulfate and ethylene oxide or propylene oxide. This is hereinafter referred to as the filtrate purification/recycle method.
Japanese Patent Disclosure No. 110,407/83 teaches a method for producing dithionites by reacting a formic acid compound, an alkali compound, and sulfur dioxide in a water-organic solvent mixture and by adding an epoxy compound, a halogenated hydrocarbon of the general formula R-X, or a mixture of two or more compounds of these types to the reaction mixture in the final stage of the reaction. Suitable epoxy compounds include ethylene oxide, propylene oxide, butylene oxide, isobutylene oxide, styrene oxide, cyclohexene oxide, epichlorohydrin, and epibromohydrin. In the halogenated hydrocarbon, R is a primary or secondary alkyl group having 1-8 carbons, an allyl group, a 2-methylallyl group, or a 2-ethylallyl group, and X is a halogen. The filtrate obtained by isolating the dithionite crystals, the organic solvent used for washing the crystals, or a mixture thereof is recycled and reused in the reaction.
The reaction is conducted according to Patent Disclosure No. 110,407/83 by dissolving sodium formate in hot water, adding methanol, and heating at 82.degree. C. under an applied pressure of 1.0 kg/cm.sup.2 gauge while stirring in a reactor equipped with a reflux condenser and a deep-cooling condenser. Subsequently, 50% sodium hydroxide solution and a methanolic solution containing methyl formate and sulfur dioxide are added simultaneously and dropwise over a 90-minute period. Stirring is continued for an additional 150 minutes at the same temperature and pressure. Cooling to 73.degree. over a period of 20 minutes is then started, and simultaneously the epoxy compound, the halogenated hydrocarbon, or a mixture thereof is added within less than five minutes. The dithionite crystals are separated out by filtration under applied pressure with carbon dioxide and subsequently washed with methanol and then dried under reduced pressure. Both the filtrate and the washing liquid were demonstrated to be equivalent to distilled methanol as the organic solvent for producing sodium dithionite. This is referred to hereinafter as the mother liquor cooling/purification method.
In European Patent Publication No. 68,248 and in U.S. Pat. No. 4,388,291, a process is disclosed for producing anhydrous dithionites in which the washing liquid discharged from the washing step is sequentially divided into two portions, a first discharge liquid and a second discharge liquid, the former being treated to convert undesirable substances inhibiting the production of dithionites into substances which do not exert an adverse influence on the production of dithionites by adding an organic compound selected from the group consisting of compounds represented by Formulas I and II and cyclohexene oxide. Formula I is as follows: ##STR2## wherein R.sub.l is hydrogen, an alkyl group containing from 1 to 8 carbon atoms, a halogenated alkyl group containing from 1 to 2 carbon atoms, a phenyl group, or a substituted phenyl group. The compound represented by this formula includes ethylene oxide, propylene oxide, butylene oxide, epichlorohydrin, epibromohydrin, and styrene oxide. Formula II is as follows: EQU R.sub.2 --X,
wherein R.sub.2 is a primary or secondary alkyl group containing from 1 to 8 carbon atoms, an allyl group or a 2-methylallyl or 2-ethylallyl group, and X is a halogen atom. Suitable compounds include methyl iodide and allyl chloride.
In the examples, the reaction of the first discharge liquid (48 parts) with the treatment compound (0.07-0.13 parts) occurred at 25.degree.-45.degree. C. for 1-24 hours after filtration at 73.degree. C. A portion of the treated first discharge liquid was mixed with nearly twice as much of the untreated second discharge liquid and used to prepare sodium dithionite after adjusting for the amount of water in the discharge liquid mixture. The resulting purities and yields for the sodium dithionite product were substantially identical to those obtained with pure methanol. This is hereinafter referred to as the washing liquid purification/recycle method.
The use of a thiosulfate-reactive compound for destroying thiosulfate ions in the mother liquor, the reactor filtrate, or the washing liquid before re-use of the methanol therein, as respectively taught in Japanese No. 110,407/83, Japanese No. 28,397/75, and European No. 68,248, enables the initial reaction mixture of the next run to be substantially free of thiosulfate ions, but it does nothing to diminish the formation of thiosulfate ions during the dithionite-forming reaction. The use of such a thiosulfate-reactive compound in the final (i.e., cooling) stage of the formate/SO.sub.2 reaction, as taught in Japanese No. 110,407, similarly enables the reaction filtrate or the washing liquid to be utilized for making sodium dithionite at yields and purities equal to results from manufacture with distilled methanol. Again, however, such protection for the next batch provides no help for the current batch.
Accordingly, there is clearly a need for destroying thiosulfate ions as they are being formed within the reaction vessel in order to minimize destruction of the sodium dithionite product. Moreover, if the thiosulfate ions could be at least partially destroyed in the current batch, the treatment needed for the mother liquor during the cooling period, for the filtrate, or for the wash liquid according to prior art methods could also be decreased before each recycle of filtrate and/or wash liquid to the next batch. Similarly, an in situ treatment to diminish the harmful effects of thiosulfate and other deleterious sulfur compounds would allow the use of raw materials containing these contaminants in the manufacture of sodium dithionite.